All the documents cited herein are incorporated by reference.
Polyesters such as, for example, poly(ethylene terephthalate) (“PET”) and poly(tetramethylene terephthalate) (“PBT”, also known as “poly(butylene terephthalate)”), generally referred to as “polyalkylene terephthalates”, are a class of important industrial polymers. Recently, poly(trimethylene terephthalate) (“PTT” or “3GT”), also called polypropylene terephthalate, has achieved commercial importance because of its elasticity, as measured, by its elastic recovery and resilience. These polyesters are widely used in shaped articles including thermoplastic fibers, films, and molding applications.
Polyalkylene terephthalates can be produced by transesterification of a dialkyl terephthalate ester with a glycol or by direct esterification of terephthalic acid with the selected glycol, followed by polycondensation. Catalysts can be used to catalyze the esterification or transesterification, and catalysts are generally used in polycondensation.
Catalysts known in the art include titanium and zirconium compounds. Organo titanium and organo zirconium compounds are well known catalysts for esterification, transesterification and polycondensation. Organic titanates, such as tetraisopropyl and tetra n-butyl titanates, are known to be effective polycondensation catalysts for preparing polyalkylene terephthalates in general, and frequently are the catalyst of choice. Use of such catalysts for polyester production results in formation of organic titanium compounds which impart color to the polymer. Thus, increasing catalyst concentration to increase rate in a titanium-catalyzed esterification, transesterification, or polycondensation can negatively impact color.
Combinations of titanium and zirconium compounds as catalysts to effect polyester production are known. For instance, U.S. Pat. No. 5,120,822 discloses a process to prepare polyesters containing a non-vicinal aliphatic diol, e.g., 1,4-butanediol or 1,3-propanediol with a dicarboxylic acid or equivalent in the presence of a titanate and zirconate catalyst. The ratio of Ti atoms to Zr atoms is 1.3:1 to 9:1. U.S. Pat. No. 6,043,335 and WO 00/46271 disclose a combination of titanium, zirconium, and phosphorus compounds for preparing a polyester, wherein the ratio of Zr:Ti is 1-5:1 and P compound:Ti is 1-25:1. A. Greiner, “Study of Mixtures of Titanium- and Zirconium Alkoxides as Catalysts in Polycondensation of PET” (Institute of Man-Made Fibers, Denkendorf, 1999) describes experiments conducted using mixtures of titanium- and zirconium alkoxide catalysts, such as tetra-n-butyl-titanate and Tyzor® NPZ tetra-n-propyl-zirconate (E. I. du Pont de Nemours and Company, Wilmington, Del., hereinafter “DuPont”) catalysts.
U.S. Pat. No. 5,981,690 discloses a process to prepare a poly(alkylene) terephthalate using a combination of titanium and zirconium complexes and an organo phosphorous compound.
U.S. Pat. No. 6,372,929 describes an organometallic compound for use as a catalyst for the preparation of polyesters comprising the reaction product of an orthoester or condensed orthoester of titanium, zirconium or aluminum, an alcohol containing at least two hydroxyl groups, an organophosphorus compound containing at least one P—OH group and a base. The base is generally an inorganic base such as sodium hydroxide, but organic bases, such as tetrabutyl ammonium hydroxide are mentioned.
WO 01/56694 describes esterification catalysts which are a complex of a first metal selected from the group consisting of titanium or zirconium, a second metal selected from the group consisting of germanium, antimony or tin, and a carboxylic acid. Suitable sources of the first metal include tetraisopropoxy titanium, tetra-n-butoxy titanium, tetra-n-propoxy zirconium and tetra-n-butoxy zirconium. Examples of carboxylic acids are acetic, oxalic, capric, lauric, lactic, citric, malic and tartaric acids. Solubilizing compounds can be used and include dihydric alcohols, such as 1,2-ethanediol, 1,3-propanediol and 1,4-butanediol. A base can also be used in preparing the complex. Suitable bases are disclosed to include inorganic bases, such as sodium hydroxide, and organic bases, such as tetrabutyl ammonium hydroxide.
WO 00/71252 describes catalysts which comprises: (a) an organometallic compound which is the reaction product of an orthoester or condensed orthoester of at least one metal selected from titanium, zirconium or aluminum, an alcohol containing at least two hydroxyl groups, and an organophosphorus compound containing at least one P—OH group, and (b) at least one compound of germanium, antimony or tin. Component (a) can also be prepared with a 2-hydroxy carboxylic acid. This document states that reference to titanium, zirconium or aluminum orthoester or condensed orthoester is intended to include orthoesters or condensed orthoesters of more than one metal, e.g., to a mixture of titanium and zirconium orthoesters. In a preferred embodiment the organometallic compound (a) includes a base. Suitable bases are disclosed to include inorganic bases, such as sodium hydroxide, and organic bases, such as tetrabutyl ammonium hydroxide. The catalyst is stated to be useful in preparing polyesters such as poly(ethylene terephthalate), poly(trimethylene terephthalate) and poly(tetramethylene terephthalate).
Present catalyst for preparing polyesters still have drawbacks. There is a need for a new catalyst that is more efficient. There is also a need for an efficient, stable catalyst that produces a polymer with reduced color. These and other objectives of this invention are described below.